Carbon nanotubes (CNTs), comprising multiple concentric shells and termed multi-wall carbon nanotubes (MWNTs), were discovered by Iijima in 1991 [Iijima, Nature 1991, 354, 56-58]. Subsequent to this discovery, single-wall carbon nanotubes (SWNTs), comprising single graphene sheets rolled up on themselves to form cylindrical tubes with nanoscale diameters, were synthesized in an arc-discharge process using carbon electrodes doped with transition metals [Iijima et al., Nature 1993, 363, 603-605; and Bethune et al., Nature 1993, 363, 605-607]. These carbon nanotubes (especially SWNTs) possess unique mechanical, electrical, thermal and optical properties, and such properties make them attractive for a wide variety of applications. See Baughman et al., Science, 2002, 297, 787-792.
In recent years, SWNTs have been intensively studied because of their many potential applications. A high-pressure CO (HiPco) process, where Fe(CO)5 is used as catalyst, is one of the most productive methods for SWNT production [Bronikowski et al., J. Vac. Sci. Technol. 2001, 19, 1800; Nikolaev et al., Chem. Phys. Lett. 1999, 313, 91; Thess et al., Science 1996, 273, 187-191]. However, the iron and non-nanotube carbon impurities present in the as-produced material need to be removed without damaging the SWNTs. To remove catalyst (such as iron, cobalt, and/or nickel) and obtain high-purity SWNTs, many purification methods have been reported previously. See, e.g., Hidefumi, Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1995, 276, 267; Yumura et al., Mater. Res. Soc. Symp. Proc. Novel Forms of Carbon II 1994, 349, 231; Dillon et al., Adv. Mater. 1999, 16, 1354; Bandow et al., Appl. Phys. A 1999, 67; Rinzler et al., Appl. Phys. A 1998, 67, 29-37; Dujardin et al., Adv. Mater. 1998, 10, 611; Hiura et al., Adv. Mater. 1995, 7, 275; Chiang et al., J. Phys. Chem. B 2001, 105, 1157-1161; Chiang et al., J. Phys. Chem. B 2001, 105, 8297-8301; Banerjee et al., J. Phys. Chem. B 2002, 106, 12144-12151; Borowiak-Palen et al., J. Chem. Phys. Lett. 2002, 363, 567; Farkas et al., Chem. Phys. Lett. 2002, 363, 111; Georgakilas et al., J. Am. Chem. Soc. 2002, 124, 14318-14319; Harutyunyan et al., J. Phys. Chem. B 2002, 106, 8671-8675; Hou et al., J. Mater. Res. 2001, 16, 2526; Hu et al., J. Phys. Chem. B 2003, 107, 13838-13842; Moon et al., J. Phys. Chem. B 2001, 105, 5677-5681; Niyogi et al., J. Am. Chem. Soc. 2001, 123, 733-734; Sen et al., Chem. Mater. 2003, 15, 4273-4279; Smith et al., Carbon 2003, 41, 1221; Thien-Nga et al., Nano Lett. 2002, 2, 1349-1352; Zhao et al., J. Am. Chem. Soc. 2001, 123, 11673-11677; Zimmerman et al., Chem. Mater. 2000, 12, 1361-1366. A common approach has been to use strong oxidation followed by an acid treatment. An oxidative treatment of raw SWNT material is effective in removing non-nanotube carbon and exposing the metal catalysts by removing carbon coating. However nanotubes can be lost or damaged during the oxidation process.
As a result of the foregoing, the current art of purifying single-wall carbon nanotubes has had limited success with regard to minimizing damage to the nanotube sidewall, and, concurrently, optimizing carbon yield and reaction time. Accordingly, it would be desirable to have a scalable purification method that removes only carbon impurities and metal catalysts—without damaging the nanotubes being purified, or wherein such damage is held to a minimum.